Description (adapted from the Application). Little is known regarding the molecular interactions between hepatitis C virus (HCV) and its natural host, the human hepatocyte. Knowledge of these interactions is critical to understand HCV pathogenesis, the ability of HCV to persist in the host, and the mechanism by which HCV evades the interferon response. With the advent of a primary hepatocyte culture system that is permissive to infection by cloned HCV RNA, the tools necessary to investigate the response of human hepatocytes to HCV infection are now available. In Project 2, it is proposed to use microarrays, consisting of over 25,000 human cDNAs, to identify cellular genes that are differentially expressed in response to HCV infection. The hypothesis to be tested is that HCV disrupts specific signal transduction pathways, including the interferon-signaling pathway, leading to alterations in cellular gene expression. This disruption of cellular signaling is likely to contribute to HCV pathogenesis and the widespread resistance of HCV to interferon therapy. In Specific Aim 1, cDNA microarrays will be used to perform a comprehensive analysis of gene expression during prolonged culture of primary human hepatocytes. In addition, the transcriptional response of hepatocytes to treatment with interferon will be examined. The data generated from these studies will further characterize the hepatocyte culture system described in Project I and will serve as a baseline for subsequent experiments analyzing HCV-infected cells. In Specific Aim 2, the changes in cellular gene expression that occur in cultured hepatocytes in response to infection with full-length or mutant HCV RNA clones will be analyzed. This analysis will be performed in the presence and absence of interferon, yielding information on the ability of HCV to alter normal interferon signaling.